1. Field of the Invention
The present invention relates to a frequency converting circuit and a receiver used in a radio LSI, for example.
2. Background Art
A frequency converting circuit (mixer) provides the output signal with a frequency determined by the frequencies of two or more input signals.
For example, a Gilbert cell mixer is a conventional frequency converting circuit (see Japanese Patent No. 3288723, for example). The Gilbert cell mixer converts the frequency “Fin” of a received signal according to the frequency “FLO” of a local oscillation (LO) signal and outputs an output signal “Fout”. The conversion gain Vout/Vin from the input to the output is expressed as gm*R. The character “gm” represents the transconductance of an MOS transistor that receives the received signal at the gate thereof. The character “R” represents the resistance of a resistor connected between a power supply and an MOS transistor that receives the local oscillation signal at the gate thereof.
To change the conversion gain of the Gilbert cell mixer described above, the resistance “R” or the transconductance “gm” has to be changed. To change the transconductance “gm”, the current or the width of the MOS transistor has to be changed.
However, if the current is changed, there arises a problem that the operating point of the MOS transistor changes. On the other hand, if the width of the MOS transistor is changed, there arises a problem that the input capacitance viewed from the preceding stage changes.
Thus, the conversion gain of the conventional frequency converting circuit cannot be precisely controlled.
To solve the problem, a conventional Gilbert cell mixer additionally has a gain controlling stage for changing the conversion gain (see Japanese Patent No. 3288723 described above).
However, according to the conventional technique, since the transistor used in the gain controlling stage is added, the Gilbert cell mixer is hard to operate at low frequencies, and the linearity is deteriorated.
In addition, as for the relationship between the voltage and the conversion gain controlled by the gain controlling stage, the conversion gain varies exponentially when the voltage varies linearly.
As described above, according to the conventional technique, the conversion gain cannot be accurately and precisely controlled.